1. Field of the Invention
The invention relates to a process for the production of tertiary magnesium phosphate octahydrate and in particular to a process for the production of highly pure crystalline tertiary magnesium phosphate octahydrate.
2. Related Art
It is known to use tertiary magnesium phosphate (a/k/a tribasic magnesium phosphate or trimagnesium phosphate), or an alkali metal pyrophosphate as a stabilizer for dibasic calcium phosphate.
It is well known that hardness and particle shape make dibasic calcium phosphate suitable for use as a base for dentifrices. The required properties of a base for a toothpaste are that, even when kept in the tube for a long time, it does not harden, remains homogeneous, does not form a coagulated mass, and does not separate into a liquid and solid phase. However, the dihydrate of dibasic calcium phosphate, when used as a base for a toothpaste is so unstable that it often tends to harden, coagulate, causing the toothpaste to separate into a liquid and a solid phase. The water of crystallization of dibasic calcium phosphate is normally thermally unstable and tends to evaporate readily when the compound is allowed to stand in dry air at room temperature, leaving the anhydrous salt behind.
With the advent of the use of monofluorophosphate additives in toothpaste formulations another problem was encountered. It was found that the monofluorophosphate components would react with the dicalcium phosphate whereby the monofluorophosphate component was converted from a water/soluble form to a water insoluble form. Since the beneficial effect of monofluorophosphate additives in toothpaste is understood to be derived principally from the water-soluble form, it has become important to develop toothpaste formulations which permit an effective amount of monofluorophosphate component to remain in the water-soluble state.
The term "monofluorophosphate compatibility" has been used as a term-of-art to describe the tendency of such formulations to permit the monofluorophosphate component to remain in the water soluble state.
The monofluorophosphate compatibility of a particular formulation may be determined by a variety of methods. Preferably, the monofluorophosphate compatibility of a formulation is determined by actually preparing the toothpaste formulation, comprising a fluoride stabilizer, placing it in storage for a predetermined period of time under controlled conditions, and then determining the amount of water-soluble monofluorophosphate which remains in the formulation. Alternatively, a simulated formulation, such as the dicalcium phosphate dihydrate to be tested, glycerine, a known amount of a monofluorophosphate component, such as sodium monofluorophosphate and a stabilizer can be "quick aged" by maintaining it at an elevated temperature for one or more hours, and then determining the amount of water-soluble monofluorophosphate remaining after such conditioning. There are, of course, many other methods for measuring the relative monofluorophosphate compatibility of various samples of dicalcium phosphate dihydrate.
The prior art teaches that dicalcium phosphate dihydrate may be stabilized by adding a small amount of an alkali metal pyrophosphate or tertiary magnesium phosphate to the mother liquor, at a controlled pH, during the preparation of the dicalcium phosphate. Specifically, it is taught that after precipitation of the dicalcium phosphate in the mother liquor, a small amount of alkali metal pyrophosphate or tertiary magnesium phosphate should be added and the entire slurry then heated for a short period of time, while maintaining the pH of the mother liquor above 7.
The alkali metal pyrophosphate or tertiary magnesium phosphate coats the surface of the dicalcium phosphate such that the coating significantly eliminates the reaction of the dicalcium phosphate with the monofluorophosphate thereby resulting in a dicalcium phosphate which remains in the water soluble state.
The effect of tertiary magnesium phosphate as a stabilizer for dibasic calcium phosphate varies greatly with the method used for its production.
Tertiary magnesium phosphate containing 0, 4, 8, and 22 molecules of water of crystallization has so far been reported, but only the octahydrate is used as a stabilizer for dicalcium phosphate. It is also known to use tribasic magnesium phosphate as a fertilizer because of its high P.sub.2 O.sub.5 content, and as an antacid. It has also been disclosed to use tribasic magnesium phosphate as a water insoluble neutralizing agent in the growing of acid producing bacteria cultures.
The known production methods for the octahydrate include one in which an aqueous solution of magnesium sulfate and dibasic sodium phosphate is made weakly alkaline with sodium bicarbonate and is then allowed to stand. Another method is one in which dibasic magnesium phosphate is boiled for a long time in a large quantity of water.
It has been disclosed to produce Mg.sub.3 (PO.sub.4).sub.2.8H.sub.2 O by the dehydration of Mg.sub.3 (PO.sub.4).sub.2.22H.sub.2 O or by the hydrolysis of Mg.sub.2 HPO.sub.4.3H.sub.2 O.
It has also been disclosed to produce Mg.sub.3 (PO.sub.4).sub.2.8H.sub.2 O by reacting orthophosphoric acid, added dropwise, to magnesium oxide powder or magnesium hydroxide powder with energetic stirring; see Japanese Patent Application No. 39-57557/1964. This order of addition, i.e., the acid to the base, is disclosed to be necessary to produce the trimagnesium but not the dimagnesium phosphate. However, it is common to produce trimagnesium phosphate by this method, which product is contaminated with unreacted Mg(OH).sub.2 or with dimagnesium phosphate. These undesired components lead to caking. They may also alter the functional properties of the trimagnesium phosphate. Furthermore, the addition of the very strong acid to Mg(OH).sub.2 has to occur at a very slow rate, otherwise the pH falls very quickly forming the dimagnesium phosphate. Therefore, this process takes several hours, normally 10-14 hours, to complete.